Why he needed a law from Congress(declaring a national emergency) is anyone’s guess. The POTUS has the plenary power under the Constitution to secure our borders. And none of the other idiot enviro laws, etc supersede the POTUS’ authority over national security.
I would guess POTUS needed a law from Congress in order to reallocate funds to an Emergency. He has the obligation to defend the country but that could be limited to temporary actions like troop deployments. To move the funds as he intends from the DEA, DoD etc to private contractors for a special (and related) purpose, I presume the office of the POTUS needed the National Emergencies Act.